An optical fiber conventionally includes an optical core, which transmits an optical signal, and an optical cladding, which confines the optical signal within the optical core. To that end, the refractive index of the core, nc, is greater than the refractive index of the cladding, ng (i.e., nc>ng). An optical fiber is generally characterized by a refractive index profile that associates the refractive index (n) with the radius (r) of the optical fiber: the distance r with respect to the center of the optical fiber is shown on x-axis and the difference between the refractive index at radius r and the refractive index of the optical cladding is shown on y-axis.
Generally speaking, two main categories of optical fibers exist: multimode optical fibers and single-mode optical fibers. In a multimode optical fiber, for a given wavelength, several optical modes are propagated simultaneously along the optical fiber, whereas in a single-mode optical fiber, the higher order modes (hereafter called HOMs) are strongly attenuated.
Single-mode optical fibers are commonly used for long-distance applications, such as access networks. To obtain an optical fiber capable of transmitting a single-mode optical signal, a core with a relatively small diameter is required (typically between 5 microns and 11 microns). To meet requirements of high bandwidth applications for access networks (e.g., 10 Gbps), standard single-mode optical fibers require the use of a modulated single-mode laser emitter for transmission at a wavelength of 1550 nanometers (nm).
Multimode optical fibers are commonly used for short-distance applications requiring a high bandwidth, such as local area networks (LANs) and multi-dwelling units (MDUs), more generally known as in-building networks. The core of a multimode optical fiber typically has a diameter of 50 microns (μm) or 62.5 microns. Multimode optical fibers have been the subject of international standardization, which define criteria of bandwidth, numerical aperture, and core diameter for a given wavelength. The OM3 and OM4 standards have been adopted to meet the demands of high-bandwidth applications (typically 10 Gbps) over long distances (a few tens to a few hundreds of meters), such as in the Ethernet high speed transmission networks. The OM3 standard requires, at a wavelength of 850 nanometers, an efficient modal bandwidth (hereafter called EMB) of at least 2,000 MHz·km to ensure error-free multimode transmissions of 10 Gbps up to a distance of 300 meters. The OM4 standard requires, at a wavelength of 850 nanometers, an EMB of at least 4,700 MHz·km to ensure error-free multimode transmissions of 10 Gbps up to a distance of 400 meters.
The most prevalent multimode optical fibers in telecommunications are the graded-index optical fibers. A graded-index refractive index profile can help to reduce intermodal dispersion (i.e., the difference between the propagation delay times or group velocity of the optical modes along the optical fiber) and achieve a high modal bandwidth for a given wavelength.
For the development of an optical home network, the choice of optical fiber is important. Multimode optical fiber is a cost-effective solution for optical data networks. With their wider numerical aperture, larger core diameter, and low modal dispersion provided by their graded-index core profile, multimode optical fibers can efficiently support 10 Gbps optical signals emitted by cost-effective, light-source-based solutions (such as Vertical Cavity Surface Emitting Laser or VCSEL), whereas single-mode optical fibers require expensive and tolerant single-mode transceivers. In particular, the connection of the light source to the single-mode optical fiber (e.g., launching conditions) requires tighter alignment tolerances than with the multimode optical fiber.
Nonetheless, an optical home network is expected to successfully connect to outside access networks, which mainly use single-mode technology because of longer reach requirements. Thus, the problem of interoperability with single-mode optical fibers needs further consideration.
In practice, multimode optical fibers are not designed to be interconnected with single-mode optical transmission systems. A home network can be seen as a network of optical fibers that enables users to connect devices at both ends of the network. Today, these devices are likely to implement multimode-transmission-based technologies that require multimode optical fibers, but tomorrow they might be designed to also operate with a single-mode-based technology. Thus, the installation of optical fiber, which is relatively costly, would have to be repeated whenever the access networks are ready to work with the optical home networks.
Therefore, it would be desirable to provide an optical fiber for a home network that can transmit both error-free multimode optical signals at an operating wavelength of the home network (e.g., 850 nanometers) and error-free single-mode optical signals at an operating wavelength of an access network (e.g., 1550 nanometers).
One proposed solution would consist of using a standard multimode optical fiber that has a refractive graded-index profile optimized for providing error-free transmission with a broad bandwidth at a wavelength of 850 nanometers. Nevertheless, when a single-mode source operating at a wavelength of 1550 nanometers is coupled to the standard multimode optical fiber, the optical signal injected in the fiber stimulates mainly the fundamental optical mode but also the optical fiber's HOMs. Indeed, a part of the optical power is coupled into the fundamental mode of the standard multimode optical fiber, and almost all of the remaining power is coupled into the set of HOMs of the optical fiber (e.g., corresponding to parasitic signal or optical noise). Because the different modes have different propagation delay times and propagation constants, on the receiver side both optical signals carried by the fundamental mode and the HOMs interfere, leading to power fluctuations that degrade the quality of the optical transmission. Therefore, due to mode field mismatch at 1550 nanometers between the fundamental modes carried by a standard multimode optical fiber and a standard single-mode optical fiber, such a standard multimode optical fiber is not adapted to an interconnection with a single-mode optical transmission system.
Australian Patent Publication No. 2002/100296, which is hereby incorporated by reference in its entirety, discloses an optical fiber that includes a single-mode core part, which has a first refractive index, surrounded by a multimode core part, which has a second refractive index, finally surrounded by a cladding, which has a third refractive index. This publication does not describe the conditions for the fiber profile to enable sufficient error-free single-mode transmission at 1550 nanometers. The disclosed optical fiber further presents a relatively low bandwidth at 850 nanometers. This publication does not address the problem of reduction of noise caused by the optical fiber's HOMs.
French Patent Publication No. 2,441,585, which is hereby incorporated by reference in its entirety, discloses a single-mode or multimode optical fiber with a central single-mode core and a multimode sheath for data transmission. In particular, the disclosed optical fiber does not exhibit a refractive graded-index core profile, which is critical for high speed performances at 850 nanometers. This publication does not describe the conditions for the fiber profile to provide sufficient error-free single-mode and multimode transmission, nor does it address the problem of reduction of the optical power coupled into the optical fiber's HOMs.
Accordingly, a need exists for an optical fiber that addresses these various drawbacks.